Effects Of Simulated Nitrogen Deposition On The Growth Characteristics Of Dominant Species In Mixed Shrub-grass Communities Of The Changbai Mountain Tundra

Under the influence of the continuous increase of nitrogen deposition and the continuous accumulation of soil nitrogen,the herbaceous plant Deyeuxia purpurea and Veratrum oxysepalum continue to invade the Changbai Mountain tundra,forming a shrub-grass community with strong interspecific competition with the native shrub Rhododendron aureum.In this paper,we took R.aureum-D.purpurea community and R.aureum-V.oxysepalum community as the subject of the research,established four nitrogen application levels to simulate nitrogen deposition in the field.The variation of vegetation in the Changbai Mountain tundra vegetation under the background of nitrogen deposition was analyzed by studying the differences in response of dominant species in mixed shrub-grass communities to nitrogen deposition.Research indicators include plant morphological characteristics,leaf nutrient element absorption and photosynthetic characteristics,the results showed that:(1)The effects of nitrogen deposition on the morphological characteristics of dominant species in shrub-grass mixed communities were different.Nitrogen deposition increased the leaf thickness,leaf dry weight,and plant height of D.purpurea,which promoted its growth.The low and medium nitrogen deposition levels significantly promoted the growth of V.oxysepalum,while high nitrogen deposition inhibited it.The morphological characteristics of R.aureum in the two shrub-grass mixed communities have different responses to nitrogen deposition.In the R.aureum-D.purpurea community,the dry weight,leaf thickness,and plant height of R.aureum reached the maximum at low and medium nitrogen deposition levels;the dry leaf weight,stem thickness,plant height of R.aureum in the R.aureum-V.oxysepalum community reached the maximum at medium nitrogen deposition levels,the increase of leaf dry weight and plant height was greater than that of R.aureum-D.purpurea community.The influence of nitrogen deposition on the morphological characteristics of R.aureum is more significant in R.aureum-V.oxysepalum community.(2)The effects of nitrogen deposition on the leaf nutrient elements of dominant species in shrub-grass mixed communities were different.Nitrogen deposition increased the leaf nitrogen content,leaf nitrogen content per unit area of D.purpurea,which promoted its absorption of nitrogen.Low and medium nitrogen deposition levels promoted the absorption of nitrogen of V.oxysepalum,while high nitrogen deposition levels inhibited it.The leaf nutrient elements of R.aureum in the two shrub-grass mixed communities have different responses to nitrogen deposition.The nitrogen deposition resulted in the increase of nitrogen content and the decrease of carbon and phosphorus content in the leaves of R.aureum in R.aureum-D.purpurea community;in the R.aureum-V.oxysepalum community,the contents of nitrogen,carbon and phosphorus in the leaves of R.aureum increased under low and medium nitrogen deposition levels.The nitrogen deposition had a stronger effect on the accumulation of nutrient elements in the leaves of R.aureum in the R.aureum-V.oxysepalum community.(3)The effects of nitrogen deposition on the photosynthetic gas parameters of dominant species in shrub-grass mixed communities were different.Nitrogen deposition increased the net photosynthetic rate and stomatal conductance of D.purpurea,and promoted its photosynthesis.The net photosynthetic rate and stomatal conductance of V.oxysepalum were significantly increased at low and medium nitrogen deposition levels,while high nitrogen deposition levels were suppressed.Low and medium nitrogen precipitation on average led to an increase in the net photosynthetic rate of R.aureum in the two mixed communities,but the net photosynthetic rate of R.aureum in the R.aureum-V.oxysepalum community increased more and the promotion effect was more significant,while high nitrogen deposition inhibited it.(4)The effects of nitrogen deposition on the chlorophyll fluorescence parameters of dominant species in shrub-grass mixed communities were different.Nitrogen deposition increased the maximum photochemical efficiency,actual photochemical efficiency,and photochemical quenching coefficient of D.purpurea,and promoted the use of light energy by D.purpurea.At low and medium nitrogen deposition levels,the maximum photochemical efficiency and actual photochemical efficiency of V.oxysepalum increased significantly,which promoted the conversion of light energy by V.oxysepalum,while high nitrogen deposition levels inhibited.Low and medium nitrogen precipitation on average led to an increase in the maximum photochemical efficiency and photochemical quenching coefficient of R.aureum in the two mixed communities,but the increase of R.aureum in the R.aureum-V.oxysepalum community is even greater,while high nitrogen deposition inhibited it.(5)In conclusion,nitrogen deposition had the most significant promotion effect on the growth of the invasive plant D.purpurea,and V.oxysepalum had a significant effect at low and medium nitrogen deposition levels.Low and medium nitrogen deposition could promote the growth of R.aureum in the two mixed communities on average,but the promotion effect is greater in the R.aureum-V.oxysepalum community.The results showed that the increase of nitrogen deposition changed the scarcity of resources in the tundra zone,and promoted the growth of vegetation within a certain range of nitrogen deposition.However,the invasive plant D.purpurea preferred nitrogen and had greater competitive advantage than V.oxysepalum,which led to the promotion effect of nitrogen deposition on the R.aureum in the R.aureum-D.purpurea community was less than that the R.aureum-V.oxysepalum community.Therefore,once the D.purpurea invades the tundra,nitrogen deposition maybe increase its competitiveness,leading to its rapid growth and expansion.The expansion of D.purpurea will increase the vacant niche in the tundra zone in spring,which was beneficial to the seed implantation and growth of V.oxysepalum,and changed the local community structure.However,when the nitrogen deposition reaches a certain threshold,the vegetation changes in Changbai Mountain tundra need to be further monitored.